Pulse code modulation communication system



April 21, 1953 R. CARBREY 2,635,159

PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 13 Sheets-Sheet l wan ma -g Q IE L 0 Z mm a N L ]T"- \0 56k K K E INVENTOR' R.L.CARBREY A TTORNEV April 21, 1953 R. L..-CARBREY PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed 001:. 30, 1947 13 Sheets-Sheet 2 NMM L RWMIL INVENTOR BYR. L. CARBREY ATTORNEY R. L. CARBREY 2,636,159

PULSE CODE MODULATION COMMUNICATION SYSTEM 13 Sheets-Sheet 3 A 7'TORNE Y l/VVE/VTOR BYE. L. CARBREY April 21, 1953 Original Filed Oct. 30, 1947 Aprilizl, 1953v R. CARBREY 2,636,159

PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 15 Sheets-Sheet 4 IVY AAA w s i r 0 P a R g *5 l I 4 R 2k *0 SN INVENTOR R. L. CARBREY ATTORNEY April 21, 1953 R. L. CARBREY 2,636,159

PULSE.CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 13 Sheets-Sheet 5 TIM/NAL EOPT IHVE/VTOR B); R- L.CARBREY ATTORNEY April 21, 1953 R. L. CARBREY 2,636,159 PULSE CQDE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 13 Sheets-Sheet 6 ATTORNEY A ril 21, 1953 Original Filed Oct. 50, 1947 R. L. CARBREY PULSE CODE MODULATION COMMUNICATION SYSTEM 13 Sheets-Sheet '7 INVENTOR 5: 1.. CARBREY ATTORNEY A ril 21, 1953 R. L. cAaaRh-v 2,636,159

PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 15 Sheets-Sheet 8 I i/wslvron BY R L. CARBREY ATTORNEY April 21, 1953 R. CARBREY ,63

PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 50, 1947 13 Sheets-Sheet 9 INVENTOR YR. L. CA RBRE Y ATTORNEY April 21, 1953 R. L. CARBREY PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 50, 1947 v rfr: 0., T;

15 Sheets-Sheet 10 INVENTOR R. L. 0A RBRE Y ATTORNEY April 21, 1953 R. L. CARBREY PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 13 Sheets-Sheet 11 1.. c4 RBREY ATTORNEY April 21, 1953- R. L. CARBREY 2,636,159

PULSE CODE MODULATION COMMUNICATION SYSTEM Original Filed Oct. 30, 1947 13 Sheets-Sheet l2 INVENTOR RL. GARBREY .4 TTORNEY April 21, 1953 R. L. CARBREY PULSE CODE MODULATION COMMUNICATION SYSTEM 13 Sheets-Sheet 15 Original Filed Oct. 30, 1947 INVENTOR RL. OARBRE Y Br ATTORNEY Patented Apr. 21, 1953 PULSE CODE MODULATION COMMUNICA- TION SYSTEM Robert L. Carbrey, Madison, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application October 30, 1947, Serial No.

783,187, now Patent No. 2,610,295, dated September 9, 1952. Divided and this application April 12, 1951, Serial No. 220,695

7 Claims.

This invention relates to coding devices for communication systems employing pulse code modulation and is a division of my copending patent application Serial No. 783,187, filed October 30, 1947, now Patent 2,610,295, September 9, 1952.

In communication systems utilizing what is known as pulse code transmission, a speech wave or other signal to be transmitted is sampled periodically to ascertain its instantaneous amplitude. The measured instantaneous amplitude is expressed by pulse codes analogous to telegraph codes.

One code which conveniently may be employed in pulse code transmission involves permutations of a fixed number of code elements each of which may have any one of several conditions or values. An advantageous code of this type is the so-called binary code in which each of the fixed number of code elements may have either of two values. One advantageous way of representing these values is to represent one by a pulse sometimes referred to as an on pulse and the other by the absence of a pulse sometimes referred to as an off pulse. Alternatively, one value may be represented by a positive pulse and the other by a negative pulse. The total number of permutations obtainable with the binary code is proportional to 2 where n is the number of code elements employed.

Because the total number of different amplitudes which may be represented by such a code of a fixed number of elements is limited, it is found desirable to divide the continuous range of amplitude values of which the transmitted signal is capable into a fixed number of constitutent ranges which together encompass the total range. Each of these smaller or constituent amplitude ranges may then be treated as if it were a single amplitude instead of a range and is represented by an individual one of the permutations of the code. In the use of this method of code transmission the instantaneous amplitude ascertained by a sampling operation is represented by the respective permutation indicative of the amplitude range, or step, which most nearly approximates the amplitude of the measured sample. If, for example, the sample amplitude is nearest to that amplitude represented by the ninth step of the signal amplitude range the permutation code corresponding to range 9 is transmitted.

It will be observed that each code element in one of its values represents the presence in sampled amplitude of a particular fixed portion of the total amplitude range, while in the other value it represents the absence of that same portion.

The groups of code pulses representing the successive sample amplitudes of the signal wave may be transmitted over a communication link to a remote station and employed thereat to reproduce or control the regeneration of the complex signal wave to which they correspond. It is usual in order to obtain the maximum utilization of such a communication link to employ a single link for transmission of a plurality of different messages by the use of the time division multiplex principle. In accordance with this principles, the messages in a plurality of channels are sampled, code groups of pulses corresponding to the sample amplitudes for each channel are generated and are transmitted in turn either in sequence or interleaved over the same communication link.

Objects of the present invention are to pro- Vide simplified coding circuits capable of high speed operation and generally to improve the reliability of the component circuits of pulse code modulation communication equipment.

In accordance with the invention, coding circuits are provided in which networks of resistors and constant current devices are employed to produce code groups in response to an applied amplitude sample.

The above and other features of the invention, will be described in detail in the following specification taken in connection with the drawings in which:

Figs. 1 and 2 are diagrams showing the manner in which Figs. 3 and 4 and Figs. 5 through 13, respectively, may be arranged to show details of the present invention;

Figs. 3 and 4, when arranged as shown in Fig. 1 comprise a schematic block diagram of the transmitting station of a pulse code modulation system in accordance with the invention;

Figs. 5 through 13, when arranged as shown in Fig. 2, form a schematic circuit diagram of the system shown in the block diagram of Figs.

3 and 4; and

Figs. 14 and 15 are graphs of wave forms appearing in various portions of the pulse code modulation system of the invention.

In the illustrative system described herein, provision is made for the accommodation of four message channels and a six-unit binary code is employed. Assuming that each of the four channels is to be employed for the transmission of speech, it is sufficient to sample each channel at a sampling rate of 8,000 samples per second. I

four equal periods, allowing 31.25 microseconds nizing signal when required and the remaining six intervals correspond to the denominational orders of the six-elementbinary'code, each of these denominational orders being represented by a pulse or by the absence'rof alpulse -.as: re-

quired according to the code representation' of the amplitude of the message sample :appliedxto that channel.

Ordinarily it is suflicient to-transmit :aisynchronizing signal only once ,for ea'chllframe. In

the present system this signal is transmitted at.

the beginning of the frame in the first 4.46- microsecond interval of the first channel and the initial 4.4fi-microsecond intervals of the remainingfzthree 4111811111818 are left :unused. Ihe :syn-

chronizingrs'i'gnal employed in the system of the present invention may comprisePtwoa-pulseef the sachet-duration as the codeu'alement pulses separated fby .:an :interval *equ'al-"to :the :duration of arcade element pulse. .Thisparticulartypeof synchronizing :signal is .zchosensbecause :it may be employed in "systems ihavingvradioptransmitters capable'iof. generating-only one "k'in-d of "output zpulse. Thus, both synchronizing and :code element pulses :may convenientlytbe one microsecondzpulses, 'the'separationiof .the' synchroniz-- ing signal from :thercode clement-signals"bein efiectedat the receiver byv-means 'to -bB1dBSfib8d hereinafter.

Graph 122 2011. .in 'llig. .114. is'a: representation of several frames of a typical pulse codefsig'nal :as transmitted.- over the :system. J The synchronizing qeulses for the .first framezappear .at .2202 and those; of' the .second frame atlz'llil. 'It will be noted that in this-graph the pulse codes for each of thesfour channels iareshownias'repree Thezgeneral-arrangement of thesynchronizing r system-elements at the transmitter :is zshown in the block diagram comprising Figs. 3 and-4 of the drawings arranged :as :indicated in'lFig; 1. Essentially. the synchronizing :system :comprises an-za-kilocyclemaster oscillator 400 located :at the transmitter and arranged to "control "as' a slave-a similar -8-kilocycle oscillator at the-receiver, through control -.circuits -=utilizing double synchronizing pulses transmitted. at the beginning of each frame of the multiplex signal.

-For-the purpose of generating these double synchronizing pulses, theB-kilocycle sinusoidal output wave of master=oscillator-400 at thetransmitter .is amplified andapplied to clipper stages 402 which transform the sinusoidal wave into a square wave of the same fundamental frequency. Thisisquare wave is. ap lied. to a .di'iferentiator 40R and lthepositiveipulses.corresponding to the leading edges of -the ,p'ositivei hali cycles ofthe square wave are acted upon "byclipper stages 40B'to obtain1-mi'crosecond'pu1ses. "The output of clipper stagesflufi is applied'toitwo different paths in' one of which "theJSUCCGSSiVe' l-microsecond pulses are passed throughfa z 'microsecond-delaynetworkuna. The delayed pulses from amplitude thereof.

separate message channels.

4 network 408 are amplified and appear on conductor-*l:0, -while the undelayed-xpulses .-from the other output .ofaclipperestages 405 appear on conductor M2. The delayed and undelayed :pulses are applied, respectively, to the delayed and undelayed pulse mixers 4| 4 and 4 l 6 by which they are'combined'and the double synchronizing pulses so obtained are combined with the code =elernent Xpulsesrfrom-coding equipment to be described 'ihereinafter. 'The composite multiplex .isignahascomprising'Fthe synchronizing and code element pulsescombined in the proper time relationship, isappli'edthrough an amplifier 302 .to-the .inters'ta'tiontransmission means which baa transmission line or as shown herein,

a radio transmitter 304. The time relationship ing circuits driven by master oscillator 46!! which control the operation of the: codingyequipmentln such fashion that the double [synchronizing pulses occur the frame in :the' timer'pcsition shownin graphiZZ 5100f Fig. :14.

The-output waves: from oscillator 490 at the transmitter are. employed to coordinate the-opera- 131011201: the messagetransmitting ,portions of the system. Briefly themessage transmitting equipment includes a time division :multiplex unit for sampling the message signals in-each ofya plurality of input channels .in .turn and-applying a sample signal for each channel to coding equipment which is arranged -to;-produce wfor 'each sample *a pulse code qgroup..representative :of :the At the completion of the coding :operation' the output 'of --the=coder .-is applied to radio frequency equipment for. transmission to the receiver. The various elements of the system referred tovabove will now :be considered in moredetail.

Transnn'tt'ing equipment In the illustrative system shown in Figs.-3-and 4., refer-red to-above, :provision is made for four In each of these channels, the message is applied to terminal eguipmentsfifi, which may include transducers,

suitablelevel adjusting devices-and. impedance transforming circuits and the like.

It will :be understoodthat the numberofcode elements employed-tocovera given volume range is determina'tive of .the granularity of reproduction .of .the message.-:signals. Accordingly, it is not desirable to transmit extremely wide volume range signals using ccdegroups havinga relatively small number of code elements. Thislimitationcan .be. reduced through the. useof .some method of volume compression. Iz" :ascontemplated. in the case ofthe system described herein, .thesystem is to be-employedonly.for voice communication, the required reduction of volume range may .be obtained through the .use. of circuits known. :as voice operatedigainadjnsting devices or vogads. :Such. vogads are well-known in the art, a typical .device ofthis. type being shownin the sole Ifigure of Patent 2152709 to L. JSchoth April 4,1939. If it is desiredto'tran'smit music over the system some "form of controlled compression at the transmitter and expansion at'thereceiver'maybe used.

In the illustrative "system, a vogad 308 fis'provided at the transmitter for each message'channel. "'Iheoutputs of the four vogads are applied through 'bufier amplifiers *3 It to atime division multiplex distributor. This distributor "samples each of the four' channels in turn and applies ranged effectively to add a direct-current component to the input message signals. This component may have an amplitude equal to at least one-half the total possible range of amplitudes to be accepted by the system. Accordingly, all signals appearing in the common output circuit will be of positive polarity. As shown herein the distributor comprises four sampler units 330 and certain control equipment. Each of these sampler units 330 may comprise a gated vacuum tube which is arranged to produce an output representative of the input signal applied thereto only when suitably enabled by auxiliary equipment. As arranged in the present system each of the audio sampler units 330 will produce an output only when there is coincidence between a channel input signal, a channel gate signal and an enabling pulse.

The channel gate signals are produced and applied to the four audio samplers in turn by a gate distributor which may conveniently be of the type disclosed in the copending application Serial No. 646,455, filed February 8, 1946, in the name of L. A. Meacham now Patent No. 2,486,491, November 1, 1949. In this gate distributor four single-trip multivibrators 332, each having a natural period equal to slightly more than one channel period are connected in a chain circuit. The first of these multivibrators is triggered by an 8-kilocycle pulse of l-microsecond duration obtained from pulse clipper 406 of the synchronizing equipment. The single square pulses in the output of each of the single trip multivibrators of the gate distributor are precisely terminated through the use of gate cut-ofi" pulses occurring at the rate of 32 kilocycles per second obtained from the synchronizing equipment shown in Fig. 4. For this purpose, the delayed 8-kilocycle pulses from delay network 403 are employed to trigger a 32-kilocycle multivibrator 4l8. Two square wave outputs 180 out of phase are taken from multivibrator M8, and applied through differentiators 420 and 42l respectively, to clippers 422 and 423 to obtain two series of positive pulses. For the sake of convenience, these two series of pulses will be referred to hereafter as 32-kilocycle 180 pulses and 32-kilocycle pulses respectively. The 32-kilocycle 180 pulses are applied to all of the single trip multivibrators 332 in parallel. These pulses serve to cut ofi the multivibrator which is producing a gate pulse at the time of application and thus to initiate the operation of the multivibrator which is to form the gate pulse for the next succeeding channel.

Referring now to Fig. 15, the 8-kilocycle pulses which initiate the operation of the chain circuit of the gate distributor are shown at 2400. The delayed 8-kilocycle pulses from delay network 408 are shown at 2402 while the 32-kilocycle 180 output pulses from clipper 422 are shown at 2404. The single square wave output signals from single trip multivibrators 332 are shown by graphs 2406 through 24 I0 and 24 l l. The first single trip multivibrator 332 of the chain produces the output wave shown in graph 2406. The trailing edge of the single square pulse is accurately located by the first 32-kilocycle 180 cut-off pulse 2M5 of graph 2404. It will be noted that the termination of the gate pulse of graph 2406 occurs simultaneously with the initiation of the gate pulse for graphs 2408, etc. Thus it will be recognized that gate pulses are applied to each of the four samplers 330 in turn.

It will be recalled that samplers 330 produce an output only when a message signal, a gate 6 pulse and an enabling pulse are applied thereto simultaneously. Accordingly a series of enabling pulses occurring at a repetition rate of 32 kilocycles per second is applied to all of the samplers in parallel, the pulses of this series being timed to occur during the B-kilocycle gate pulses. At

any particular time only the sampler 332 which is in receipt of a channel gate pulse is enabled by the 32-kilocyc1e enabling pulses to produce an output signal, the amplitude of which is proportional to that of the message signal applied to the associated terminal equipment. The necessary 32-kilocycle enabling pulses are obtained from clipper 425. The pulses from clipper 425, shown in graph 2414, are applied to a Z-microsecond delay network 424, the delayed pulses from which are shaped by a clipper 426 to obtain the pulses shown in graph 24! 6 of Fig. 24. The parallelled outputs of the four samplers are applied through an amplifier 334 to the coding equipment.

Briefly, the coding equipment is arranged to perform in the following manner. First, the sample obtained from the common output of the samplers 330 and representative of the instantaneous amplitude of the message signal in one of the four channels is stored for the duration of a coding cycle. Subsequently, voltage steps corresponding respectively to A, A and $4 of the total amplitude range to be accommodated by the system are successively added to the stored sample. After the addition of each step the sum of the stored sample and the voltage step or steps added thereto is compared with a reference voltage equal to the maximum amplitude range to be accommodated. If the summation voltage is greater than the reference voltage a code element pulse is produced for transmission. Upon the production of a code element pulse, the voltage step which was last added to the stored sample to produce the pulse is removed from the summation. If on the other hand, the summation voltage is less than the reference voltage no pulse is produced. In either event, the next voltage step is then applied and is added to the stored sample (plus any preceding steps which were not of sufiicient amplitude to cause production of a code element pulse). The comparison operation is repeated, etc.

Considered in more detail the coding operation is carried out as follows. First, voltage step one which is equal in amplitude to one-half the maximum peak-to-peak signal to be accommodated by the system is added to the stored sample. It will be recalled that a bias voltage was added to each of the message signals in the sampling equipment to make all signal voltages positive. Accordingly, the maximum peak-to-peak input signal corresponds to the maximum amplitude range of the coder. If the stored sample happens to be of amplitude less than one-half the maximum peakto-peak amplitude, the summation of the stored sample and the first step signal does not exceed the reference voltage and no code element pulse will be produced. If on the other hand the signal is greater than one-half the maximum peak-to-peak amplitude a code element pulse will be produced and the first step signal will be removed leaving only the stored sample.

Next, the second voltage step corresponding to' one-fourth of the maximum peak-to-peak amplitude is added to the stored signal plus the first step, if present. If the amplitude of the stored sample is less than one-fourth the maximum peak-to-peak amplitude the summation of the stored sazmple, the first step -o-one=ha1= ampli timer-and the-second step of ionefourth- .-amp1itudeiwill remain flees-thanthe referencewcxltage. Accordingly, no :code element 'ipuiSe WiII-iibB: produccd andiihe :secoridstep-wiliremain. "if on the othernhand, ::the stored =.samp1e isgreaterthan one fourth but *lessthan oneehaif :the total ipeake to-peeikiamplitude e node-element. pulse ,.;will; be producedsinoe thesummationof the-message signal and; the twosteps; that .is, thepne-half andtrthe. .oneeriounthgamplitude stepswill exceed the reference mltage.

ilihis process is repeated for each of .thesmaller steps, rea ch step corresponding to and ioontrolling production-ore di it in one of ithe denominational the shinanyrnode: group'iiozbe aproduced atithe ioutputof .ithe oofiingiequipment.

:Ecg,liiiciinerit:for:performing. the operations mitlinedrabmtefis, shown in iblockifiorm in Fig." 3. messegessi hal :sampie; :iromwithemultiplex .tdistributor is applied to a sample storage devicefiflfi cnmoaisingza eathcdeioilower;tubeitheyacezthode cincuitiof which includes. aarsterase capacitor-3:38 andathezcazthode resistorfiflflgoi a :se. 60nd cathode fol ower.:.3ii throuehrwhichithevolta e steps. re-

multiwibretorszbeins prev-ides;fereee hwdenominetional Qlifififjfif thecbinary code; groups to be produced. .tflhese zfiip flQp .multivibrators are double-stability circuits each of which may .be trig ered to-eeither' ref its conditions of stability and caused (to 1 remain in tha.t -condition until a ain triggered. -;Fcr conveniencew-;of,descri tion, onezofthe two-conditions .of stability for -each;circui-t'is identifiedes the .011, conditionwhile the other is identifiedas the ,ofiiepndition. -When these. step inultiwibrators are in the off condition, cur-rent foritheeconducting tubes of the six flip-flop-step multivibrai-OES isecbtained from a common supply-.346. Thus; when the stepqmultivibrators are .in .the .50 condition, constant eurrents-are drawn irem zthissupply'through a summation networkmornprising series resistors-3.4 3, 35Q;;3.5 2,,- 35!; 356 and .31 5.8; which. have respectively ohmic ralues of 16R, 43.13.5143, 21?, .R {and R, where the voltage drop across Rismecle-lfg the total nea-krmmeakwamplituderrenge dated. .i-Th is :the resi t nce-il ed throueh which the rmul iv ra' rs fortsteosol jthlm'lghr 6: show currentewheniin the "ofFgconditionuarearespeotiW yi32R;:1fiRm R# R, and Rrendithemolt e W95. produced by switching 1the,; -.-severa1 multimbratorsxtoithe flow-conditioner 3 /2 z /i, it, sees ndiMerthe toteluemplitudetozbemcommodated.

sI'nitiai1 -y, 51821 aof :ithe step multivibrators are switched to-there wonditionnnd the steps-me switched .onirene mat-time :the 810111186 of the coding process. ;:;'l3he terminelrofrthesiimmation network remote rE-rom source .346 is connected to step :cetthede iiollower ,-.3T42., rapplying 13theretoe voitegeequalitonhat. Qfiihe-SQHIGBTfififiI minus the drop :thezsummaltion network.

:rPriorztorthegheg-imiin figs-coding ,oneraticnzthe sample storage capacitor isriischargedt-to remove any lifisidilflit icherg-e zwhichiis mresentiherein :anjcl the six steprzmultiyibraters; areztriggereditocthe ofiteondition. .ilhese imtialvconditions@aremroduced-rbyrestoration .andzreset pulsesiohtained from the srnchrcnizin icirouits. iA.-..32ki1ocyc.1e .;puise,:.derived;iimmiclim fir 425 is::anp1ied toga cspacitondischarge(tubet-360,.-1which;is onneoted -acrossrthesampleistoreeeeenacitomu.

toteaaccQmmo- 32 ;hilocyc1eiP-rpu1se, renders theidischargezituhe conductive to furnish; a. discharge path for .the storageecapacitpr.

"Two microseconds laterthe step multivibnators are restoredztc the .0 condition. For this purpose .a' 32-kilocyo1e 10 :pulse from the output of :2- microseoondadeley network 424is applied-through clippers 426: ami'i izfi .to obtain ,a series of negative pulse wwhich are rappliedthrough lead 429 in parallel :rto :all poi" the step, multivibrators 342. Suchneeative pulses trigger all of the multivibraitors :to the";0ff condition.

"The operation of the coder. .to produce a .code group .of pulses corresponding to the instantaneousramnlitude of :the message signal in the first channel; will nowbe considered. As stated above. allioi zliheisamplersiiiw ere-.enabIed'My the delayed 32-kilocycle 0 pulses shown in graph-24lz6 At the-time .cf: thezfiirsti32-kilocycie 10-". pulse, howererfloniycthe sampler forehannel twilibeinreceipt ciithesrgateqpulse. necessary :to permit the passage hf'azmesser e signal therethrough, the ate spl lse iinthis :sase. .beineihat shown atzxitt in graph; 259.6 :(Fie. 1.5).. :As :a result a voltage which: is a .meesureof the instantaneous amplitudeof themessagesignelzin channel 1 is applied t sand is-rsiored :in c pacitor .;33.8.;at ,the time of oscurrenc iof ,thezenabling. pulse. 1.46 microsecon s rl t rxthe' firs flip-flop multivibrator, 342- is turned iDn to produce -.woltege step '1, whi h epre nts one-h lf therto eil amp e. range. 'Bhis'andthe o her stepzmultiyibrators are controiled ibygia,gate distributor: which ,is similar in all respects to the channel gate distributor described above. This distributor comprises six se -s n tQr-s; 5 5 1 i fil fi i l b i PIO- rid d for-each the s x step mu t ib etors 3.42.. lihe -.distrihutcr 1 3 in operet onib delayed .3 kil c :puis s- Thes pu ses. :ar ;;obtaine from the 1outputmfrestoration pulse .clipper 428' and are further.- delayed 1146 mierosecqnds :by;:=a delay network-A30. iih se elayed pul es.ere shown iingraphiGfl-i. The .first gategenerator is turned off ,and theseccndohe on,- e tc. by a tragin of .224=ki1op yclepv,isesobtainedirome shock-excitedlsC oscillator-4,32, which is triggered by; the output puisesfrom' 1.46 .microsecond delay circuit 439,. ThenllQPut of this-oscillator is-differentiated and. clipped -in (iifferentie.t or =43?! to obtain 234.- kil cycle; pulses which ar rapp edz ara lel-t9 he six ea s genera ors 1.35 .I e. :224-lsi1ccwle pulses are sho.wn ,in -g'r a ph 242 B {and the trains f gate puisese n -et d yithe distribu or i rehn cation :to .the .sisstep .multivibrators are gfihQWXl in gr.aphs MZ-Zto 2.432. Itwill be noted theta.

blame-interval cguali tdllrai flnifi) nesat poise,-

vibrator is suddently turned cff. '(JQnSfiquently the correspondin svoltage drop throughithezsummation network :is removed; and. thewqltageaw.

plied to step cathode follower 342 is suddenly increased by one-half. This voltage step, the amplitude of which corresponds to one-half the maximum amplitude range of the coder, is added to the sample stored on capacitor 358 and the sum of these two quantities appears at the output of a cathode follower tube 366 which repeats the voltage across storage capacitor 338 and resistor 340. The output of this cathode follower is applied to a comparison circuit 310 comprising a single trip multivibrator to one of the grids of which is appled a reference voltage equal to the maximum voltage to be accommodated by the system and to the other grid of which is applied the output of storage cathode follower 368.

If the output of storage cathode follower 368 exceeds the reference voltage, the comparison circuit 310 is switched from one of its conditions of stability to the other and its output voltage is suddently increased, being switched from a low value to some higher value. The output'of comparison circuit 310 is applied to a second single trip multivibrator 312. This single trip multivbrator is provded with a reference voltage such that it will trip at the mid-amplitude f the output voltage step produced by the first single trip multivibrator. The output of the second single trip multivibrator is applied to a differentiator 314 to obtain a pulse occurring at the time at which the second single trip multivibrator is actuated. This pulse is passed through a 1- microsecond delay network 316 and is applied in parallel to all of the stop circuits 366. Each of these stop circuits comprises a gate tube and these tubes are successively enabled by the output pulses from gate pulse generators 362 at the same time that start tubes 364 are actuated. As a result only the stop circuit associated with the step 1 multivibrator will pass the delayed pulse from single trip multivibrator 312 and differentiator 314. This pulse returns the step 1 multivibrator to the off condition, removing the one-half amplitude step from the summation voltage applied to cathode follower 368 and consequently returning the comparison circuit 310 and single trip multivibrator 312 to their initial condition. Thus, delay network 316 determines the duration of the pulse appearing at the output of single trip multivibrator 312. This pulse, which is the code element pulse for the first denominational order of the code group is applied to pulse mixer 318 for combination with the synchronizing pulses from delayed and undelayed pulse mixers 4M and M6. It will be recognized that these synchronizing pulses occur in the first 4.46-microsecond interval of the channel frame, the trailing edge of the delayed synchronizing pulse occurring 2.46 microseconds before the production of the first code element pulse.

If the summation of the sample voltage applied to capacitor 338 and the one-half amplitude step voltage applied to step cathode follower 342 as described above is less than the reference voltage, comparison circuit 310 will not operate and as a result no code element pulse will be produced and applied to the stop circuit 366 for the step 1 multivibrator. As a consequence this multivibrato-r remains in the on condition and the half-amplitude step continues to be applied to step cathode follower 342.

At the time of the gate pulse from the second generator 362 of the code element gate distributor, start tube 364 for the step 2 multivibrator triggers this multivibrator to the on condition, cutting off the current drawn through the summation network by this circuit. As a result a step corresponding to one-quarter the total amplitude acceptable by the system is applied to step cathode follower 342. The summation of this quarter amplitude step, the half amplitude step is not removed at the conclusion of the first code element interval and the sample voltage stored on capacitor 338 is applied to the comparison circuit and produces a code element pulse in the second denominational order of the code group if the reference voltage is exceeded.

The remaining four step generators are triggered in the same fashion in the remaining four code element intervals of the first channel time and a complete code group representation of the sample amplitude is produced. The code element pulses of this code group after combination with the synchronizing pulses are applied to transmitter 304. The coder is then reset as described above and is in condition to code the sample for the second message channel, after a 4.46-microsecond interval corresponding to the blank at the beginning of the second channel frame.

. Schematic circuit diagram Certain of the component circuits forming elements of the block diagram described above will now be considered in more detail. A circuit schematic showing the detailed arrangements of a transmitting station is shown in Figs. 5 through 13 when arranged as shown in Fig. 2. Referring to this schematic, the 8-kilocycle master oscillator which regulates the operation of the entire communication system is indicated at i200 in Fig. 10. This oscillator must have a high degree of frequency stability and may conveniently be of the type disclosed in Fig. 2 of Patent 2,385,260 to L. R. Cox, September 18, 1945. In accordance with this patent a flexure type mode crystal I202 is employed to control an oscillator circuit comprising vacuum tube I204 and associated circuit components. For this purpose crystal I202 which is of the duplex plate type is connected in circuit in the following manner. One section of the crystal is connected in series with a tuning capacitor I206 between control grid of vacuum tube I204 and ground. The output of vacuum tube I204 appearing across resistors I201 and I208 connected in series between the anode of tu.e I204 and the positive terminal of battery I2"! is applied through a capacitor I2 I 2 to a divider comprising resistors I2I4 and I2 I6. An output taken from the junction of resistors I254 and I2I6 is fed back to the other section of crystal I202. This particular type of crystal is sensitive to changes in the voltage applied to it and accordingly this voltage is held constant within a high degree of accuracy by compensating means shown herein as a varistor I2I8 connected in series with a capacitor I220 across the output of vacuum tube I204. The varistor tends to hold the output voltage of the. tube constant, while the combination of the varistor and capacitor I220 acts to produce a voltage phase shift in response to changes of the gain of the amplifier which will compensate for the voltage phase shift caused in the crystal as a result of changes in the voltage applied thereto.

The output of crystal oscillator I200 is applied to a tuned amplifier comprising a pentode I222 which has a tuned circuit including inductor I224 and resistor I226 connected in series in the plate circuit. This amplifier provides a sinusoidal output at a frequency of eight kilocycles.

This sinusoidal output is converted into an 8- kilocycle square wave by clippers I228 and I230,

eorupidse pentode :tubesso era'tediassoverrthe positive hali' cyoles ofithe" sinusoidal 1 input "wave: while"- the second; operating on thezinverteo the negative. hal'fic'ycles" to "prodriceaian; essentie'liy: square? wave output;

A} cireuitcomprisingnarseriescapacitcrctnt and a shuntie'sistor Ilsein:tlietdiithfltwficlihgerrtflll -difierehtiates theisquare waste 'outputa'therefrom and the output oi iithezdifierentiating ;'comprising:alternatezpositive andnegativespulses is appliedi td 'ez-thirdzclipper: tz llimhiclroperatess as an overdriveziamplifierzi v finlytthenegaltlvespulses oftheoifferentiated wave lrave'anycefiect .ortzthe plate current of: this clipper 2116313118565 are irr- Vested; appearing assra'; series-oi positiveepulses-in theplatecircuit; Thesepulsesareifiirthemshaped in eathode follower I mmtheoutputloi which-con:- prise'saiseriesof il microsecondpulsess. ltiwilllbe noted that: since output conneetionsi: .ameotaken from both theplate and cathodelotthis tube both positive and negative l mi'crosecond 8-kilocy-cle pulse-series are-avaiiasieforuse in-the-varieus portionsoftheequipment;-

The positive i microssoond pmses attained from the: cathode of clipper 1242' are applied to a delay rietworie M et which introducesa dslay of twoffi-icroseconds: The delayed pulsee ere applied to airisolatioiia-mplifier I582 which may qomprise psnwse-tuoe: Positivepuises taken fronT-th oathod otthis tulo'e are applied topen tecie MM appearing as negative pulses across lead resistor" Ito's-:- Uz idelayed positive pulses froiiitliecathodecf clipper: I2 lfl ai" flee-applied to the-control grid of pentodetube" Milli-employ iiig load' resistor M083 iircommon-with the pentode mes; Thus,-= theundelayed i-microseoond pulse from vacuum tube IZM-appears-as a negas tive pulse across load iresistor N98, while a. second negative i ni-icros'econdpulse: from delay network I m appears acrossthe same resistor two mieroseconos later. These-two" pulses; s'ep ereted twomicroseconds; comprise the syn ohronizing' signal-"employed to control the operatldn of'the slave osoill'atorat the receivin station. For this mposethe ootput acrosslaeire sistermac; which pentodes momma 4Uo" share a thirs pentode Himin the output' oi the signaling circuit, is applied to" an amplifiercomprising pentod'e eta-and powercampiifiertub'e set; The negative pulsesappearing-"across resister-Mail! areihverted to produce positive pulses iu ampli-fier eflz andcorrespondinglyappear as positive pulses iii the 'cathode-output 'of ampliiier out; Theout ut fronr vaeuum-tuhe eoaiis taic'en from thecathode through capacitor slower appli catiorr to transmitting equipment shown genmany at f 5 G82 synchronization of, transmitter circuits The roontrol pulses :required "to coordinate: the cooperation of the samplingju codi ngf and dis-- tributing; circuits at theztransm-itter are provided by a" series Fofi pulsergeneratingi and shaping; oinouits-drivenhy master oscillatorv IZIlil;

The 8-k-ilocyoleyl microsecond' pulses-appeariiigia-t-the toutput of-amplifier I 48'2 are-employed to: control: the' several: circuits. required? to r gemerate' pulsesof the e-requirediphlarities".andirepee tition rates ior: 11585111 ;theevarious:circuits'of :the transmitter; i Accordingly; the: negative: i pulses from the shodeecitcilite amplifier: I402 sweetne- 1'2 ployed td-triegen a multivibratortunedrto432M10- cycles: This multivibrator, whichcomprisesthe two sections: of a dual triodetube I300 connected in a: conventional'- circuit, provides two.- output wavesioi-essentially-squarewaveform. Rheoutput fromtheplate of the right-handsection "of multivibraton tube 13W is: diiferentiated through the action of seriescapacitor I382 and shunt resistor-= mi l-to produce a: series ofwpulsesof. al-' ternate: polarity: The negative pulses: o= this series correspond" to thev leading edges' of the square-rwavevpulses 'producedbyv the-multivibrabrator I 300' isdifferentiated by series= capacitor I3I6 -and shuntresisto-r I308.-- and the differentiated wave is i clipped in'pentode I 3 In" to produce aseriesvof positive output pulses ocourringat-the rate of 32,000 per second and 180 out of phase with the 32-kilocycle 0- puises-= produced I atthe output" of: clipper. I312. The pulses produced by clipper I-3II1 will: be referred-to hereinaiterras 32-kilocycle" 180 pulses. The 32-kilocycle 180 pulses are: applied to the input time division gate generator: Negative-az-kiiocycle 0" pulses from the: plate f amplifier I31mare-alsoinverted-and amplified in tri'ode: I3I9to-obtain positive. 32- kilooycle 0 pulses which are applied. to dischargethe storage capacitor of thecodingequipmerit.-

The' positive pulses-fromthe cathode: of amplifier 'I 3Hi-are2app1ied to a 2 microsecond delay network I500; the-: output of which is reshaped Icy-amplifier I502; The negative pulses -fromkthe plateof amplifier I502:- are appliedto thesamplingftubesin thednput circuits QfJtheEfQurchannels-and serve to gatethrough the input signal of the sampling=tube which is at the same time enabled byan' output pulse from the-input time division multiplex. distributoin The. output-of amplifier I502 isalso applied to inverter an'iplifier 15M; Amplifier IEIM-is connectedto operate asxa icathodeefollower-with cathode: resistor. I 000 and*thenegativcmulses therefrom are applied in parallel tOa-thesi-X flip flop circuits: of the coder to"restore-theserclrcuits'to the off condition priorto'the initiation of the coding operation; Positive pulses'i appearing in the plate circuitof amplifier I504 are applied to a delay network -I 55$:Whl0h introduces'a delay of L46 microsecondsand the delayed pulses are applied to a cathode -1ollowencomprising triode I508; Thepositive--32=kilocycle 0 pulses appearing; across the load resistor I 244 ofthis cathode-follower are employed to *initiate the operation of the code element'gateidistributor: These pulses are-also-employedlto shook excitean LC oscillator comprising triode-I'SIB, the cathode" circuit of which-in eludes inductor I5 I 2 and I capacitors I5 I 4 and I5I6, the combination ofthese'elements being tuned 1 to the frequency: of 224 kilocycles': The output iof this oscillator is taken through a: cathode:follower I BI 81 to I provide a high impedance load' and thus' -to' avoid-loading 'd'own rthetuned circuit; The 224leki1ocyc1e sinusoidal warewpipearing across load resistor I520 of cathode follower II8 is clipped in clipper stages I522 and I524 to obtain a 224-kilocycle square wave which in turn is diirerentiated by the combination of capacitor I526 and resistor I528 and clipped in triode clipper I530 to obtain a series of positive pulses occurring at a repetition rate of 224,000 per second. These pulses are applied to the code element gate distributor to terminate the operation of the gate output pulses from each of the gate generators.

Gate distributor circuits As pointed out in the description of the block diagram the present communication system employs several gate distributor circuits. These circuits are arranged to produce series of gate pulses occurring in timed relationship and made available in separate circuits. Thus in the transmitter a 4-stage gate distributor is employed to control the input time division multiplex circuit. Six-stage gate distributor circuits are also employed at the transmitter to control the operation of the coding equipment.

Conveniently all of these gate distributor circuits may be of the same type and may, for example, be of the general type disclosed in copending application, Serial No. 646,455, filed February 8, 1946, in the name of L. A. Meacham, now Patent No. 2,486,491, November 1, 1949. Since all of the gate distributors may be of the same type it will be suificient to describe only one of them in detail. By way of example, a 6-stage gate distributor shown in the upper portions of Figs. 10 and 12 is employed to control the operation of the coder at the transmitting station. This distributor comprises six single trip multivibrators I246, I248, I250, MID, H2 and "4 each including the two sections of a dual triode tube. the anode of the left-hand section of the tube is connected through a capacitor to the grid of the right-hand section while the two cathodes are connected together and through a common load resistor to ground. These six multivibrators are connected in a chain circuit with the plate of the left-hand section of each of the dual triodes capacitor-coupled to the grid of the lefthand section of the succeeding dual triode in the chain. Positive bias voltages obtained from potentiometers i252 and I254 for the first sta e and from potentiometers I4I6 and I 4I8 for the remaining stages are so adjusted that the grid potentials of the right-hand sections of the tubes are higher than those for the left-hand sections. Accordingly, current normall tends to flow in the hight-hand section of each tube. This current flow increases the voltage drop across the common cathode resistor for each stage and thus assists in preventing current flow through the left-hand section of the tube.

Operation of the distributor is controlled by two sets of pulses shown in graphs 2401 and 2420 of Fig. 15. The first of these sets of pulses, having a repetition rate of 32 kilocycles, is applied only to the first stage of the chain and controls the initiation of a cycle of operation. The second set of pulses, having a repetition rate of 224 kilocycles, is applied in parallel to the cathodes of all of the stages of the chain circuit and is employed to control precisely the duration of each of the gate pulses- As may be seen by comparison of graphs 2401 and 2420 which are drawn on the same time base the gate cut-on pulses of gr ph'242il divide the interval between In each of these single trip multivibrators the control pulses of graph 2401 into seven equal periods. It will be noted, however, that only six gate generators are provided. This particular distributor is employed to control the coding equipment and only six code pulses are required, the seventh interval being left blank for the synchronizing signals.

It will be convenient to consider the operation of the circuit first as affected only by the control pulses, it being assumed for the moment that no gate cut-ofi pulses are applied to the cathodes of the code generator stages. Assuming therefore that all of the single trip multivibrators are in their condition of permanent stability (in which current flows through the right-hand section of each of the dual triode tubes), operation of the distributor is initiated by a positive control pulse applied to the grid of the left-hand section of tube I245. Upon application of this positive pulse, the grid potential of the left-hand section of tube I 246 is made suificiently positive with respect to the cathode thereof to permit the initiation of current flow through that section of the tube. Immediately the anode potential of the left-hand section of the tube is reduced causing the coupling capacitor to discharge. This applies a negative potential to the grid of the right-hand section of the tube such that current flowing through this section is cut off. In the absence of the gate cut-off pulses and after an interval determined by the relative values of the coupling capacitor connected between the anode of the left-hand section and the grid of the right-hand section of the tube and the resistor connected in the grid circuit of the righthand section, the potential applied to the grid of the right-hand section returns to such a value that plate current will again begin to flow through this section of the tube. The voltage drop across the common cathode resistor is increased in response to this current flow to such an extent that the left-hand section of the tube is cut ofi, the control pulse having terminated during the period in which the left-hand section of the tube was conducting.

The anode of the left-hand section of single trip multivibrator I246 is coupled to the lefthand section of single trip multivibrator I248 through an RC circuit which tends to diiferentiate the square wave output appearing at the anode of single trip multivibrator I246. Accordingly, the derivative of the output of the lefthand section of single trip multivibrator I246 will control the operation of the second single trip multivibrator of the chain. Thus when current flow is switched from the right-hand section to the left-hand section of single trip multivibrator I246, a negative pulse is applied to the grid of the left-hand section of single trip multivibrator I248. Since this section is already cut cit, this negative pulse will have no effect upon the operation of the second single trip multivibrator. Upon c-ompletion of the cycle of the first single trip multivibrator, however, current flow is switched back from the left-hand section to the right-hand section with the result that a, positive pulse is applied to the control grid of the left-hand section of single trip multivibrator I248. This positive pulse triggers the second single trip multivibrator which then goes through a cycle of operation identical with that of the first single trip multivibrator.

The RC circuits of each of the six single trip multivibrators may be adjusted to control the duration of the six gate pulses produced'by the 

